In many medical procedures, probes, such as endoscopes and catheters, are inserted into a patient's body. Various methods of determining the location of these inserted medical devices are known in the art. X-ray imaging is the most commonly used location confirmation system. Position determining systems can also be used for this purpose.
Ultrasound intrabody position sensing is well known. Some such systems require an active transducer in the catheter, generally at or adjacent the distal end of the catheter, connected by wires to a console outside the body. The transducer either receives ultrasonic waves from emitters outside the body or radiates ultrasonic waves to receivers outside the body. Other ultrasonic systems use a passive ultrasound reflector in the catheter which gives a strong reflection of ultrasonic waves irradiating the body without the necessity of running wires through the catheter. These passive systems necessarily create a strong background of ultrasonic radiation against which the position of the reflector must be found.
U.S. Pat. No. 3,713,133 to Nathans, whose disclosure is incorporated herein by reference, describes a theft-prevention system in which a piezoelectric crystal having a resonant frequency is incorporated into a device which is then attached to individual items within a store. When a radio frequency (RF) signal having a frequency equal to the resonant frequency of the crystal strikes the crystal, an oscillating electrical field gradient is produced across the face of the crystal at the radiated RF frequency, and two tin foil members mounted on the crystal vibrate, emitting ultrasound. Detection of the ultrasound under appropriate conditions produces an alarm, indicative of an attempt to remove the item from the store without authorization.
U.S. Pat. No. 3,713,133 also describes a small, thin metal diaphragm having a resonant vibrational frequency. When the diaphragm is irradiated with an ultrasound field at or near the resonant frequency, the diaphragm vibrates at that frequency. An RF field also irradiates the diaphragm at a substantially higher frequency, and the vibration of the diaphragm induced by the ultrasound field modulates the RF field. This modulation is detected by an RF transducer, which activates an alarm. These systems do not provide specific information describing the location of the item, but only that the item has entered a detection area (typically near an exit from the store).
PCT Patent Publication WO 96/05768 to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes a locating system for determining the location and orientation of an invasive medical instrument whereby an externally applied RF field induces a current in three coils located within the invasive medical instrument. Wires or some other form of physical leads are required to carry this induced signal from the catheter to a signal processor in the extrabody space. The processor analyzes the signal so as to calculate the location and orientation of the invasive medical instrument.
U.S. Pat. No. 5,522,869 to Burdette et al., and U.S. Pat. No. 5,549,638 to Burdette, whose disclosures are incorporated herein by reference, describe an ultrasound device for use in a thermotherapy apparatus to treat cancer. To operate this device, ultrasound transducers are arranged in a cylindrical shape around the treatment site, and are activated by radio frequency power. The resultant ultrasound waves heat tissue within the treatment site, thereby producing the desired therapeutic effect. The output of this device is therefore heat, and there is no indication of the location or orientation of the individual ultrasound transducers.
U.S. Pat. No. 5,325,873 to Hirschi et al., whose disclosure is incorporated herein by reference, describes a system to verify the location of a tube or other object inserted into the body. It incorporates a resonant electrical circuit attached to the object which resonates upon stimulation by a hand-held RF transmitter/receiver external to the body. The electromagnetic field generated due to resonance of the circuit is detected by the hand-held device, which subsequently turns on a series of LEDs to indicate to the user the direction to the target. An additional visual display indicates when the transmitter/receiver is directly above the object.
U.S. Pat. No. 5,412,619 to Bauer, whose disclosure is incorporated herein by reference, describes a system for tracking the three-dimensional position of a moving object. At least three ultrasound transmitters in known positions send signals which are detected by receiving stations mounted on the moving object. At each station, the detected signals are encoded into a form suitable for radio transmission, and a radio transmitter passes the information to a computer which determines the location of the object through a triangulation algorithm. The system in U.S. Pat. No. 5,412,619 is primarily suitable to be attached to the exterior of the human body to allow determination of gross body positions and movements.
U.S. Pat. No. 4,807,202 to Cherri et al, whose disclosure is incorporated herein by reference, comprises a visual environment simulator in which three separately positioned ultrasonic transmitters send signals to each of three ultrasonic receivers in order to monitor the change in the spatial coordinates and the orientation angles of the viewer and/or mobile unit carrying the viewer.